linux-kernel - Re: [PATCHv3 4/6] tty/lockdep: Add ldisc

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Date:   Tue, 11 Sep 2018 14:01:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Nathan March <nathan@...net>,
        Pasi Kärkkäinen <pasik@....fi>,
        Peter Hurley <peter@...leysoftware.com>,
        "Rong, Chen" <rong.a.chen@...el.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Subject: Re: [PATCHv3 4/6] tty/lockdep: Add ldisc_sem asserts

On Tue, Sep 11, 2018 at 02:48:19AM +0100, Dmitry Safonov wrote:
> Make sure under CONFIG_LOCKDEP that each change to line discipline
> is done with held write semaphor.

But you don't do that. You just assert it is held, not for writing.

> Otherwise potential reader will have a good time dereferencing
> incomplete/uninitialized ldisc.
> 
> Exception here is tty_ldisc_open(), as it's called without ldisc_sem
> locked by tty_init_dev() for the tty->link.

You fail to explain how that is not broken...

> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jiri Slaby <jslaby@...e.com>
> Signed-off-by: Dmitry Safonov <dima@...sta.com>
> ---
>  drivers/tty/tty_ldisc.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
> index fc4c97cae01e..202cb645582f 100644
> --- a/drivers/tty/tty_ldisc.c
> +++ b/drivers/tty/tty_ldisc.c
> @@ -471,6 +471,7 @@ static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
>  
>  static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
>  {
> +	lockdep_assert_held(&tty->ldisc_sem);

Did you want:

	lockdep_assert_held_exclusive(&tty->ldisc_sem);

?

>  	WARN_ON(!test_bit(TTY_LDISC_OPEN, &tty->flags));
>  	clear_bit(TTY_LDISC_OPEN, &tty->flags);
>  	if (ld->ops->close)